The present invention relates to a process for the production of paper that exhibits a good alkaline wet strength and a good water absorbency, to the paper so produced and to products comprising such paper.
One field in which the present invention finds application is in the production of casing paper, which may be used for the manufacture of packaging for meat products such as sausage.
Casing paper is commonly manufactured from paper webs of relatively strong, high-tenacity natural fibres, such as abaca, siscal or flax. The paper web is saturated with a dilute viscose solution, for example a solution obtained by diluting a solution containing 7% by weight of cellulose (as cellulose xanthate) and 6% by weight of sodium hydroxide to a 1% cellulose content. The viscose-saturated web is dried and the cellulose in the viscose is then regenerated by passing the web through an acidic regenerating bath containing, for example, a 1-8% sulphuric acid solution. The web is then washed free of acid and dried to produce a paper web impregnated with acid-regenerated cellulose. This casing paper is then generally formed into rolls ("master rolls").
Casings for the packaging of processed meats, e.g. sausage, may be manufactured from the casing paper by cutting it into strips which are then folded to form tubes. The tubes are saturated with an alkaline viscose solution, containing, for example, 7% by weight of cellulose and 6% by weight of sodium hydroxide. The cellulose in the viscous is then regenerated by means of an acidic regenerating bath containing, for example, a dilute sulphuric acid and possibly such salts as sodium sulphate or ammonium sulphate. The tube is then passed through one or more baths in order to wash out the acid and the salts.
If desired, the tube may be passed through an aqueous bath which contains a plasticizer, e.g. glycerine, for the regenerated cellulose. The tube is dried by passing it through a heated chamber (the tube being in an inflated state) to give a cellulosic tubing which has embedded therein a paper web. This tubing may then be stuffed with a processed meat product under pressure. A process of this type is described in detail in U.S. Pat. No. 3,135,613.
The purpose in treating the initial paper web with dilute (1%) viscose solution, followed by regeneration, is to provide the web with strength and structural integrity so that it may withstand the treatment with the highly caustic viscose solution used in the formation of the casing tubes. The amount of acid-regenerated cellulose in the casing paper is, in fact, comparatively low; thus, the casing paper may have a typical basis weight (weight per unit area) of 20 grams/m.sup.2, of which the acid-regenerated cellulose accounts for 0.6 g/m.sup.2, compared with the material of the casing tubes which may have a typical basis weight of 70-80 g/m.sup.2, of which 50-60 g/m.sup.2 may be accounted for by the acid-regenerated cellulose. However, despite the initial treatment with viscose, the treatment with the highly caustic viscose solution used in the formation of the casing tubes will inevitably entail a certain degree of softening and weakening of the web. This imposes a limit on the production speeds if difficulty in handling the webs and possible breakdowns in production are to be avoided. There is accordingly a need in the art for casing paper having an improved alkali resistance in order to permit higher production speeds in the manufacture of the casing tubes.
In U.S. Pat. No. 3,378,379, there is disclosed a tubular regenerated-cellulose casing for dry sausage, which casing is provided with a coating comprising a cationic thermosetting resin bonded to the inside wall thereof. The patent suggests that polyethylene imine may also be employed for this coating, although this material is not, in fact, a thermosetting resin. The purpose of the inner coating is to improve the adhesion of the sausage casing to a dry product despite any shrinkage which may occur when the dry sausage product is processed and dried in the casing over a prolonged period of time. It should be noted, however, that in the aforesaid process it is not the casing paper as such which is treated with the thermosetting resin, but the tubular casing material. In the embodiment illustrated in U.S. Pat. No. 3,378,379, the cationic thermosetting resin is applied to the inner surface of the casing tube after the application of glycerine and before the casing is dried, in an inflated state, in a heated chamber.
Another problem recognized in U.S. Pat. No. 3,378,379 is the variation in extensibility in the transverse direction exhibited by casing paper strips cut out from different parts of the master oil. This can cause variation in the properties of the final casing tubes, which may therefore be unsatisfactory to the meat packager, for whom dimensional stability in the product is of commercial importance. To meet this problem, it is suggested in the aforesaid U.S. patent that a cationic thermosetting resin (e.g., a reaction product of epichlorohydrin and a polyamide, a modified melamine-formaldehyde resin or a modified urea-formaldehyde resin) may be employed as the bonding agent in the casing paper, instead of the commonly employed acid-regenerated viscose. The cationic thermosetting resin is employed in an amount of at least 0.5% by weight based upon the dry weight of the impregnated fibrous web. The resin may be incorporated into the fibrous web by adding the resin to the fibrous slurry prior to forming the fibrous web. Alternatively, the formed fibrous web may be impregnated with the resin by passage through an aqueous solution of said resin. The said U.S. patent also discloses the use of the thermosetting resin in combination with viscose which is not regenerated with an acid; however, in the latter case the viscose may be auto-regenerated by storing the web for a sufficient period.
U.S. Pat. No. 3,484,256 discloses a process for the production of casing paper, in which a fibrous web is bonded with a cationic thermosetting resin, and a polyacrylamide resin. An interesting point is that in Table III of that patent, there is a comparison of the tensile strengths, under various conditions (dry, wet and 6% aqueous NaOH), of paper webs that are bonded with regenerated viscous or with polyamide-epichlorohydrin resin and polyacrylamide or with just polyamide-epichlorohydrin resin. The results might at first suggest that the alkaline wet strength of the web bonded with just polyamide-epichlorohydrin resin is at least as good as that of the viscose-bonded web. However, these results required careful qualification since the alkaline wet strength decreases as caustic solution soak time increases. In this connection, casing manufacture typically involves caustic soak times of at least five minutes and, more likely, twenty minutes.
In United Kingdom Patent Specification No. 1,091,105, there is described a process in which casing paper is produced by incorporating into a paper web an alkaline-curing resin such as polyethylene imine or a polymeric reaction product of epichlorohydrin and a polyamide. The use of the alkaline-curing resin in place of the customary treatment with dilute viscose is said to result in a casing paper which has more uniform characteristics across its width and which results in a casing having improved burst strengths. However, it has since been found that the casing paper so produced has an alkali resistance which is insufficient to permit sufficiently high production speeds for the commercial manufacture of the casing tubes.
In general, the alkali resistance of regenerated viscose casing paper is merely adequate; this means that in order to avoid numerous breaks in the web on the manufacturing equipment, the operation speeds must be limited. It has also been found that the absorbency of viscose-treated casing paper webs deteriorates with aging. This will also serve to restrict the rate of production of the casing tubes; it is not uncommon for a period of three months or more to elapse between the manufacture of the casing paper and its subsequent use.
Another field in which the present invention finds application is in the production of paper for the manufacture of infusion pouches, for example tea bags and spice bags.
Tea bags and the like are commonly formed as pouches of a material ("tea bag paper") that is permeable to water and to the beverage formed by infusion, i.e. by the dissolution of soluble solids in the contents of the pouch, upon the application of hot water thereto. Desirable characteristics of the material are cleanliness, good absorbency, high wet strength, a sheet structure to permit rapid diffusion of the tea extract, and ability to perform satisfactorily in high-speed packaging equipment with which the tea bags are fabricated and filled. The strength of the pouch is determined, by and large, by three main factors: firstly, the fibre composition of the paper and the chemical treatments, if any, that the paper has undergone; secondly, the nature of the contents (e.g. tea) of the pouch; and thirdly, the interaction, if any, between the solids dissolved during the infusion and the chemicals with which the paper fibres have been treated.
Hitherto, the chemical treatment of fibrous webs for tea bag paper has, in general, been effected by either of two methods. In the first method, the fibrous sheet is saturated with viscose and the cellulose in the viscose is then regenerated with a dilute acid. This method produces a fibrous web with an adequate strength in aqueous alkaline conditions. However, the product has a disadvantage, in that it can impart an undesirable taste to tea or other beverages.
The second method comprises saturating the fibrous sheet with a mixture of a polyamide-epichlorohydrin resin and carboxy methyl cellulose, as described, for instance, in U.K. Patent Specification No. 1,111,165. This method produces a fibrous web which is somewhat weaker in alkaline, aqueous media than the fibrous webs produced by the first method. This product, however, is neutral as regards taste.
The drawbacks of these prior art methods are particularly noticeable when the papers are used for the production of herbal tea bags; herbal teas are alkaline. Thus the viscose-treated paper can be formed into a bag having an adequate alkaline wet strength, but which gives rise to a beverage of poor taste or flavour. On the other hand, the other type of paper will not affect the taste of the beverage, but the tea bags formed therefrom will have a low alkaline wet strength.
Mention should also be made of U.S. Pat. No. 2,698,793 which discloses a process for producing a sized cellulosic sheet having resistance to water and ink. A water-insoluble, hydrophobic sizing agent, in the form of a water-dispersible salt, and an alkyleneimine resin are added separately to an aqueous suspension of the cellulosic fibres, which suspension is allowed to stand after each addition, and the fibres are subsequently formed into a sheet. The sheet is then heated to 105.degree. to 150.degree. C. to develop the sizing properties of the sizing agent. Additionally, other sizing or filling agents, e.g. starch, can also be employed.
The said U.S. patent, it should be stressed, is concerned with giving the sheet a resistance to water by rendering it water-repellent. Thus, water and water-based inks are prevented from penetrating the interstices of the paper by the increased surface tension at the surfaces of the paper matrix. It should be noted that the sizing agents specified in U.S. Pat. No. 2,698,793 must be water-insoluble, hydrophobic compounds. The "wet strength" exhibited by the sheets treated in accordance with the process of the said U.S. patent is attributable primarily to the fact that the fibres remain in a dry state below the water-repellent surface of the matrix. It has, in fact, been known for a long time that water-resistance can be imparted to paper webs by treatment with chemicals such as rosin size. However, once water has been driven into the interstices of the web, either by mechanical forces or by use of surfactants, the apparent wet strength of the sheets would be lost.
In contrast, the present invention is directed to the very different problem of producing a paper web that has alkaline wet strength (a property with which U.S. Pat. No. 2,698,793 is not concerned) yet at the same time is absorbent. Clearly, a water-repellency mechanism, as employed in U.S. Pat. No. 2,698,793, would be of no use in the field of casing papers and tea bag papers: the former are required to absorb aqueous viscose solutions and the latter must not present any impediments to the passage of water and of the aqueous infusion.
It is an object of the present invention to provide a process for the production of paper having a good alkaline wet strength and a good absorbency.
It is a further object to provide such a process whereby casing paper can be produced which retains its absorbency even after aging, thus permitting casing tubes to be produced using high-speed commercial processes.
It is another object of the present invention to provide a process whereby tea bag paper can be produced that combines taste neutrality with a good alkaline wet strength whilst retaining a good absorbency.
It is yet a further object to provide such processes that avoid the use of viscose and its consequent acid regeneration.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
According to the present invention, a process for the production of paper comprises forming a fibrous paper web and treating the fibres with a water-soluble cationic, thermosetting, epihalohydrin-containing resin, a nonviscose film-forming material and a polyalkylene imine.
The present invention also provides paper, for example casing paper and tea bag paper, when prepared by the aforesaid process.
The invention also provides casing material (casing tubes or skins) prepared from the said casing paper by a process comprising the application of a caustic viscose solution and subsequent acid regeneration, as well as infusion pouches, e.g. tea bags (especially herbal tea bags) and spice bags, manufactured from the said tea bag paper.
A better understanding of this invention will be obtained from the following description of the process including the several steps and the relation of one or more of such steps with respect to each of the others and the article of manufacture possessing the features, characteristics, compositions, properties and relation of elements described and exemplified herein.